1. Field of the Invention
This invention relates to a resistance-type temperature control system and, in particular, a system utilizing a number of condition responsive elements as part of a high impedance network adapted to control the temperature within a zone.
2. Prior Art
Various systems operating on resistance thermometer principles to control the temperature in buildings utilizing low-pressure steam are well known and documented in the prior art. Typical are systems which are described in U.S. Pat. Nos. 2,362,977 and 2,431,790. Those two patents describe a system which utilizes as a demand sensor a temperature-sensitive resistance thermometer mounted on the inside surface of a pane of window glass in the environmental area to be controlled. Control circuitry is described which reacts to the inner surface temperature of the glass to measure the heat requirements of that area as determined primarily by prevailing outdoor conditions as well as inside air temperature and the mean radiant temperature of interior parts of the zone to be controlled. Those patents disclose the use of a pair of temperature-sensitive resistances, one mounted below and the other above the heating element in a convector type of heating unit. This heat supply sensor reacts to the rise in temperature of the airstream passing through the heating element as a measure of the rate of heat output from the heating system. A mechanically-operated potentiometer which is in synchronism with the heat control valve is utilized as feedback to the controls which adjust the motor to operate the control valve. Accordingly, in those patents, the system utilizes a potentiometer which is responsive to the degree of opening of the control valve, and by combinations of sensing circuits in Wheatstone bridge elements, a variety of automatic control operations can be achieved.
Other patents utilizing the same principle and more elaborate embodiments are shown in U.S. Pat. Nos. 3,054,562 and 4,079,887. All of these prior art patents are directed to a variety of systems which are utilized to control the temperature in buildings utilizing low-pressure steam as a basic heating medium. Another variation is shown in U.S. Pat. No. 4,072,268.
Within this basic prior art, it has long been recognized that systems operating utilizing resistance thermometer principles are exceptionally sensitive and will respond to minute electrical imbalances. As a result, various control systems have been devised operating on electrical bridge principles which have required skilled instrument technicians to insure continuity and stability of operation. However, for successful application to building temperature control, various skills and talents among operating personnel are not common, and such systems must be fabricated, installed and maintained by people who are not generally technically familiar with the intricate operating procedures of such systems.
The prior art U.S. Pat. No. 4,072,268 departs from a system utilizing Wheatstone bridge elements, but utilizes a closed-loop rebalance system having a single open-loop outdoor sensor with many general functional similarities. Its temperature sensor and the rheostat controlled by the valve motor must have substantially linear variable impedance characteristics. As shown in the prior art U.S. Pat. No. 4,072,268, the outdoor sensor is a thermistor 21 having a variable resistor 22 to provide adjustment of the thermistor so that the impedance seen by a control input of an operational amplifier 18 will be linearly variable over the range of temperatures to be sensed. One input to the operational amplifier 18, as shown in FIG. 2, then is the output of the outdoor sensor 20. A second input comes from the rheostat 15 which is responsive to positions of the valve motor. By appropriate gating from timer mechanism 33, the signals are compared by the operational amplifier 18 at certain intervals. The difference signal is fed simultaneously to two comparator circuits 26 and 27 which also receive inputs from variable resistors 39 and 40 to provide reference voltages to those comparators. One comparator 26 is used as a temperature increase comparator such that if the output of the amplifier 18 is above the null condition, the temperature increase comparator will detect that increase and apply a control signal through lamp 28 and triac 29 to effectuate a closing of the valve motor 12. Similarly, a decrease comparator 27 is used to effectuate an increase of the valve motor if the output potential from the operational amplifier 18 is detected to require an increase in the set rate at which heat is supplied to the space.
U.S. Pat. No. 4,072,268 requires a substantial linearity of signals to effectuate a linear output of the amplifier of the control system. Specifically, detailed calibrations of the thermistor 21 are required to match the equivalent impedance characteristic of the rheostat coupled to the control valve. Hence, in order for a simple comparator system to operate as shown in U.S. Pat. No. 4,072,268 to derive a linear output of the comparator amplifier, crucial equivalent impedances must be maintained. Moreover, in a more sophisticated system utilizing a compensator and heat balancer, the requirements of matching impedance will become more difficult to maintain with a system such as U.S. Pat. No. 4,072,268.
Additionally, U.S. Pat. No. 4,079,887 shows a prior art system utilizing low impedance couplings. In such a system, it was necessary to disconnect switch contacts (not shown in the patent) when an indicate circuit was to be connected to any heat sensor, demand sensor or feedback potentiometer. Accordingly, a large number of switch contacts was required to connect and disconnect different coupling circuits to each sensor. Accordingly, calibration of the system was time consuming to achieve, especially in field conditions.
Additionally, in the system shown in U.S. Pat. No. 4,079,887, voltage divider potentiometers were used in series within each temperature sensor bridge leg to select a balance point. Besides this technique, the resulting total imbalance of each indicate and control circuit would be compensated for by an adjustment. This, therefore, resulted in the different adjustments not being made independent of other variables in the circuit. Typical was the voltage divider potentiometer 108 with wiper arm 110 which is in series with the heat balancer elements 104 and 106 and compensating examples 214, 226 and 228, as shown in FIG. 2 of the U.S. Pat. No. 4,079,887.
The prior art U.S. Pat. No. 4,079,887 also required a multitude of control knobs to perform all indicate and compensator functions for the control circuits. For example, a room temperature dial 68 and a compensator dial 66 were used to perform individual temperature, compensation and other indicate functions. The dial 66 was used to determine the valve opening, rate at which heat was being supplied to the zone and compensation positions; while the temperature dial 68 was used to provide a reading of the temperature at the resistance thermometer. Accordingly, a more complicated and, accordingly, more expensive control panel was required.
Accordingly, this invention is a direct improvement of the prior art represented by U.S. Pat. No. 4,079,887.